Maleic acid-based aqueous cleaning compositions and methods of using same

ABSTRACT

Acidic aqueous cleaning compositions containing a strong acid and maleic acid are disclosed. The maleic acid provides improved surface safety.

TECHNICAL FIELD

The present invention relates to cleaning compositions forhard-surfaces. More specifically, compositions are described which giveoptimal performance in removing limescale stains and encrustations whileensuring appropriate surface safety, especially when used for cleaningand descaling metal surfaces.

BACKGROUND

Tap water contains a certain amount of solubilized ions which upon waterevaporation eventually deposit as salts such as calcium carbonate onsurfaces which are often in contact with said water, resulting in anunaesthetic aspect of said surfaces. This limescale formation anddeposition phenomenon is even more acute in places where water isparticularly hard.

It is well known in the art that limescale can be removed chemicallywith acidic solutions, and a great variety of acidic cleaningcompositions have been described for this purpose.

However, in many instances acidic compositions may cause the problemthat the acids which are used damage the surfaces being treated. Inparticular, some corrosion/staining may occur when metal surfaces suchas aluminium, chromed steel or stainless steel are treated with suchacids.

It is therefore an object of the present invention to obviate this issuein providing a cleaning composition for the removal of limescale, saidcomposition possessing a superior limescale removing capacity whilebeing also safe to metal surfaces.

It has now been found that for an acid which has a first pKa notexceeding 5, surface safety is improved without compromising onlimescale removing capacity by combining said acid or mixtures thereofwith maleic acid, in appropriate ratios. Indeed, it has been found thatthe compositions of the present invention comprising maleic acid andsuch an acid in appropriate ratios are significantly safer to metalsurfaces than the same compositions without maleic acid, while beingalso particularly effective in removing limescale.

An advantage of this invention is that said compositions are also safeto other surfaces besides metal surfaces including synthetic surfaces.

U.S. Pat. No. 3,277,008 discloses solid compositions suitable for usewith water to form aqueous solutions for descaling the internal metalsurfaces of the jacket of the glass-lined jacketed equipment. Saidcompositions comprise a cleaning agent such as sulfamic or hydrochloricacid, maleic acid and a corrosion inhibitor. Surfactants are notdisclosed.

DE-3,822,658 discloses compositions useful for removing oxide layersfrom metal surfaces like copper or bronze. Said compositions comprise amineral acid for example nitric acid, a carboxylic acid such as maleicacid, phosphonic acid and thioureas. Nonionic surfactants are disclosedas optional ingredients without specifying appropriate amounts.

EP-A-0,496,188 discloses a composition comprising nonionic surfactantstogether with maleic acid whereby good limescale removal is provided.Citric acid is disclosed together with maleic acid in some of theexamples. But citric acid is not effective in removing limescale at lowpH.

SUMMARY OF THE INVENTION

The present invention is an aqueous cleaning composition having a pH offrom 0.1 to 4.5, suitable for removing limescale from hard-surfaces,comprising from 0.01% to 30% by weight of the total composition of asurfactant or mixtures thereof, maleic acid and an acid which has afirst pKa not exceeding 5, with the exception of citric acid, ormixtures thereof, in a weight ratio of maleic acid to said acid suchthat surface safety is improved.

The present invention also encompasses a process of treatinghard-surfaces, especially metal surfaces, wherein a compositionaccording to the present invention is used in its neat or diluted form.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graphical representation of the effect on surface appearanceof an aqueous solution containing about 3% sulfamic acid containingvarious levels of maleic acid.

DETAILED DESCRIPTION OF THE INVENTION

The compositions according to the present invention are designed forremoving limescale or soils comprising limescale as an essentialcomponent. Thus they comprise as a first essential ingredient an acidwhich has a first pKa not exceeding 5 or mixtures thereof. Preferablythe acids to be used herein which are particularly efficient to removelimescale on many surfaces, have their first pKa not exceeding 4, morepreferably not exceeding 3 and most preferably below 2. According to thepresent invention said acids can be organic or inorganic acids. Examplesof inorganic acids are sulfamic acid (pKa=0.1), hydrochloric acid(pKa<0), nitric acid (pKa<0), phosphoric acid (pKa=2.1) andmethanesulfonic acid (pKa=1.9). An example of organic acid is formicacid (pKa=3.75). Preferred for use herein is sulfamic acid alone or inmixture with another acid. Furthermore, the compositions according tothe present invention are free of citric acid which does not performvery well as a limescale remover at pH below 2. The compositions of thepresent invention comprise from 0.1% to 20% by weight of the totalcomposition of an acid which has a first pKa not exceeding 5 or mixturesthereof, preferably from 0.1% to 10% and more preferably from 0.1% to5%.

The compositions according to the present invention further comprisemaleic acid as the second essential ingredient. The compositionsaccording to the present invention comprise from 0.1% to 45% by weightof the total composition of maleic acid, preferably from 1% to 25% amdmore preferably from 8% to 20%. This percentage is calculated on thebasis of the molecular weight of the acid form, but maleic anhydride isequally convenient for use in the compositions according to the presentinvention. Indeed maleic anhydride is generally cheaper and it istransformed into the acid form when incorporated in an aqueous medium.

It has been observed that surface safety is improved when treating metalsurfaces with the compositions of the present invention comprisingmaleic acid and an acid having its first pKa not exceeding 5, in anappropriate weight ratio of one to the other. Said ratio is dependent onthe specific acid used and thus is different for each acid or eachmixture thereof. According to the present invention the weight ratio ofmaleic acid to said acid is such that the surface safety is improved. By"surface safety improvement" it is to be understood that less damage tothe surface treated is observed with the compositions of the presentinvention compared to the same compositions without maleic acid, this ata given time of contact. More specifically, we have observed that byplotting the surface damage against the amount of maleic, in any givencomposition comprising an acid having its first pKa not exceeding 5, acurve is obtained which comprises a step (see FIG. 1). The surfacesafety is "improved" when the step is reached (A), preferably half thestep (B) and more preferably the whole of the step (C). A methodsuitable for measuring surface safety is a visual grading methodmentioned hereinafter in the examples. Said surface safety test methodis reported in the literature in ANSI 124.1-1980.

In a preferred embodiment of the present invention, the compositionsherein comprise sulfamic acid as said acid having its first pKa notexceeding 5. Preferably said compositions comprise from 0.5% to 5% byweight of the total composition of said sulfamic acid with a weightratio of maleic acid to said sulfamic acid of from 15:1 to 3:1,preferably of from 8:1 to 4:1.

The compositions according to the present invention have a pH of from0.1 to 4.5, preferably of from 0.1 to 3 and more preferably of from 0.5to 2.

The compositions according to the present invention comprise as afurther essential ingredient a surfactant or mixtures thereof.Preferably the compositions according to the present invention comprisefrom 0.01% to 30% by weight of the total composition of said surfactantor mixtures thereof, more preferably from 0.05% to 10%, more preferablyfrom 0.1% to 8% and most preferably from 0.1% to 3%. All types ofsurfactants may be used in the present invention including nonionic,anionic, cationic, amphoteric or zwitterionic surfactants. It is alsopossible to use mixtures of such surfactants without departing from thespirit of the present invention.

Suitable nonionic surfactants to be used herein are alkoxylated alcoholnonionic surfactants which can be readily made by condensation processeswhich are well known in the art. However, a great variety of suchalkoxylated alcohols, especially ethoxylated and/or propoxylatedalcohols is also conveniently commercially available. Surfactantscatalogs are available which list a number of surfactants, includingnonionics.

Accordingly, preferred alkoxylated alcohols for use herein are nonionicsurfactants according to the formula RO(E)e(P)pH where R is ahydrocarbon chain of from 2 to 24 carbon atoms, E is ethylene oxide andP is propylene oxide, and e and p which represent the average degree of,respectively ethoxylation and propoxylation, are of from 0 to 24. Thehydrophobic moiety of the nonionic compound can be a primary orsecondary, straight or branched alcohol having from 8 to 24 carbonatoms. Preferred nonionic surfactants for use in the compositionsaccording to the invention are the condensation products of ethyleneoxide with alcohols having a straight alkyl chain, having from 6 to 22carbon atoms, wherein the degree of ethoxylation is from 1 to 15,preferably from 5 to 12. Such suitable nonionic surfactants arecommercially available from Shell, for instance, under the trade nameDobanol® or from Shell under the trade name Lutensol®. These nonionicsare preferred because they have been found to allow the formulation of astable product without requiring the addition of stabilizers orhydrotopes. When using other nonionics, it may be necessary to addhydrotopes such as cumene sulphonate or solvents such asbutyldiglycolether.

Suitable anionic surfactants for use herein are according to the formulaR₁ SO₃ M wherein R₁ represents a hydrocarbon group selected from thegroup consisting of straight or branched alkyl radicals containing from6 to 24 carbon atoms and alkyl phenyl radicals containing from 6 to 15carbon atoms in the alkyl group. M is a salt forming cation whichtypically is selected from the group consisting of sodium, potassium,ammonium, and mixtures thereof.

Other suitable anionic surfactants can be represented by thewater-soluble salts of an alkyl sulfate or an alkyl polyethoxylate ethersulfate wherein the alkyl group contains from 6 to 24 carbon atoms, andpreferably from 1 to 30 ethoxy groups for the alkyl polyethoxylate ethersulfates.

Suitable cationic surfactants to be used herein include derivatives ofquaternary ammonium, phosphonium, imidazolium and sulfonium compounds.Preferred cationic surfactants for use herein are according to theformula R₁ R₂ R₃ R₄ N⁺ X⁻, wherein X is a counteranion, R₁ is a C₈ -C₂₀hydrocarbon chain and R₂, R₃ and R₄ are independently selected from H orC₁ -C₄ hydrocarbon chains. In a preferred embodiment of the presentinvention, R₁ is a C₁₂ -C₁₈ hydrocarbon chain, most preferably C₁₄, C₁₆or C₁₈, and R₂, R₃ and R₄ are all three methyl, and X is halogen,preferably bromide or chloride, most preferably bromide. Examples ofcationic surfactants are stearyl trimethyl ammonium bromide (STAB),cetyl trimethyl ammonium bromide (CTAB) and myristyl trimethyl ammoniumbromide (MTAB).

Suitable zwitterionic surfactants contain both cationic and anionichydrophilic groups on the same molecule at a relatively wide range ofpH's. The typical cationic group is a quaternary ammonium group,although other positively charged groups like phosphonium, imidazoliumand sulfonium groups can be used. The typical anionic hydrophilic groupsare carboxylates and sulfonates, although other groups like sulfates,phosphonates, and the like can be used. A generic formula for somepreferred zwitterionic surfactants is

    R.sub.1 -N.sup.+ (R.sub.2)(R.sub.3)R.sub.4 X.sup.-

wherein R₁ is a hydrophobic group; R₂ and R₃ are each C₁ -C₄ alkyl,hydroxy alkyl or other substituted alkyl group which can also be joinedto form ring structures with the N; R₄ is a moiety joining the cationicnitrogen atom to the hydrophilic group and is typically an alkylene,hydroxy alkylene, or polyalkoxy group containing from 1 to 4 carbonatoms; and X is the hydrophilic group which is preferably a carboxylateor sulfonate group. Preferred hydrophobic groups R₁ are alkyl groupscontaining from 8 to 22, preferably less than 18, more preferably lessthan 16 carbon atoms. The hydrophobic group can contain unsaturationand/or substituents and/or linking groups such as aryl groups, amidogroups, ester groups and the like. In general, the simple alkyl groupsare preferred for cost and stability reasons.

Other specific zwitterionic surfactants have the generic formulas:

    R.sub.1 --C(O)--N(R.sub.2)--(C(R.sub.3).sub.2).sub.n --N(R.sub.2).sub.2 (+)--(C(R.sub.3).sub.2).sub.n --SO.sub.3 (-)

or

    R.sub.1 --C(O)--N(R.sub.2)--(C(R.sub.3).sub.2).sub.n --N(R.sub.2).sub.2.sup.(+) --(C(R.sub.3).sub.2).sub.n --COO(-)

wherein each R₁ is a hydrocarbon, e.g. an alkyl group containing from 8up to 20, preferably up to 18, more preferably up to 16 carbon atoms,each R₂ is either a hydrogen (when attached to the amido nitrogen),short chain alkyl or substituted alkyl containing from one to 4 carbonatoms, preferably groups selected from the group consisting of methyl,ethyl, propyl, hydroxy substituted ethyl or propyl and mixtures thereof,preferably methyl, each R₃ is selected from the group consisting ofhydrogen and hydroxy groups and each n is a number from 1 to 4,preferably from 2 to 3, more preferably 3, with no more than one hydroxygroup in any (C(R₃)₂) moiety. The R₁ groups can be branched and/orunsaturated. The R₂ groups can also be connected to form ringstructures. A surfactant of this type is a C₁₀ -C₁₄ fattyacylamidopropylene(hydroxypropylene)sulfobetaine that is available fromthe Sherex Company under the trade name "Varion CAS sulfobetaine"®.

Suitable amphoteric surfactants are surfactants which are similar to thezwitterionic surfactants but without the quaternary group. However, theycontain an amine group that is protonated at the low pH of thecomposition to form cationic group and they may also possess an anionicgroup at these pHs.

In one embodiment of the present invention where it is desirable to givesome viscosity to the compositions of the present invention thesurfactant is a mixture of a nonionic surfactant as describedhereinbefore together with a cationic surfactant as describedhereinbefore. Said compositions comprise from 0.5% to 15% by weight ofthe total composition of said mixture of surfactant.

The compositions according to the present invention are aqueous.Accordingly, the compositions according to the present inventioncomprise from 10% to 95% by weight of the total composition of water,preferably from 50% to 90%, most preferably from 70% to 85%.

The compositions according to the present invention may further comprisea variety of other ingredients including perfumes, colorants,bactericide, thickeners, dyes, chelants, pigments, solvents,stabilizers, corrosion inhibitors and the like.

In one embodiment, the compositions of the present invention are free ofcorrosion inhibitors, i.e. compounds which have the sole purpose ofinhibiting corrosion.

The compositions according to the present invention are particularlysuitable for treating metal surfaces which can be found in a kitchen orin a bathroom. Indeed, the compositions of the present invention exhibitgood limescale removing properties for both the kitchen-type stains andthe bathroom-type stains, i.e. for stains which contain not only calciumcarbonate but also soap scum and/or grease.

The compositions according to the present invention are also suitablefor treating metal surfaces which can be found in appliances such as forexample irons, coffee machines or kettles. The compositions to be usedin the application of descaling appliances preferably contain low levelsof surfactants, preferably below 1% by weight of the total compositionand more preferably from 0.1% to 0.9%. In the application of descalingappliances it is preferred to use the cationic surfactants describedhereinbefore as the surfactant.

The present invention further encompasses a process of treatinghard-surfaces, especially metal surfaces, wherein a composition ashereinbefore defined is dispensed in its neat form from a container ontosaid surfaces, then left to act onto said surfaces and then removed byrinsing. Said process can be used both for treating metal surfaces foundin bathrooms, kitchens or appliances.

The present invention further encompasses a process of treatinghard-surfaces, especially metal surfaces, wherein a composition ashereinbefore defined is used in diluted form. The expression "used indiluted form" herein includes dilution by the user, which occurs forinstance in the application of descaling appliances. Typical dilutionlevels are of from 0.5% to 50% of the compositions. The compositionsherein are also particularly suitable to be used in hot conditions, e.g.when descaling a coffee machine said compositions can be used dilutedand in hot conditions (80° C. to 180° C.).

As used in the foregoing paragraphs, the expression "treating" mayinclude washing as the compositions according to the present inventioncomprise surfactants and removing limescale while improving surfacesafety due to the appropriate combination of acids of the presentinvention.

The present invention is further illustrated by the followingexperimental data and examples.

EXPERIMENTAL DATA

1)

The surface safety of solutions comprising 3% of sulfamic acid (pKa=0.1)together with increasing percentages of maleic acid is evaluated by thefollowing surface safety test method.

The surface safety test method is conducted on an aluminium surface. Adrop of the solution to be tested is put onto said surface and leftthere for 16 hours. In this test the drop of the solution is left to dryat room temperature (20° C.). At the end of the exposure time, thesurface is rinsed with soft water and wiped dry. The same surfacewithout any treatment is taken as a reference. The comparison betweenthe surface treated and untreated is done visually using the followinggrading scale:

0=no visual surface damage

1=slight surface change/damage; weak staining

2=slightly more surface damage; medium staining

3=lots of surface damage; strong staining.

As a result the curve in FIG. 1 was obtained reporting the visualsurface damage against the increase of the percentage of maleic acid byweight of a total solution comprising 3% of sulfamic acid.

The curve in FIG. 1 shows that surface safety is improved when maleicacid and sulfamic acid are present in appropriate ratios of one to theother.

2)

Further examples of compositions according to the present invention arethe following. These compositions are made comprising the listedingredients in the listed proportions (weight %).

    ______________________________________                                        Ingredients:  Compositions                                                    (% by weight) 1        2     3      4   5                                     ______________________________________                                        Maleic acid   8        8     14     14  14                                    Sulfamic acid /        /      2     2   2                                     Nitric acid   /        0.5   /      /   /                                     Formic acid   4        /     /      /   /                                     Lutensol AO7* 3        3      3     0.3 /                                     Cetyl trimethyl                                                                             /        /     /      /   0.3                                   ammonium bromide**                                                            Waters & Minors                                                                             up to 100                                                       Exposure (hours)                                                                            22       5     17     24  24                                    ______________________________________                                         *Lutensol AO7 is a nonionic surfactant.                                       **Cetyl trimethyl ammonium bromide is a cationic surfactant.             

The surface safety test method as described hereinbefore has beenconducted for compositions 1 to 5, on an aluminium surface and astainless steel surface with an exposure of different time period asindicated hereinbefore. No visual difference has been found for thecompositions 1 to 5 when comparing the surfaces treated with thereference surface (untreated). In other words, these compositions aresafe to both the aluminium and stainless steel surfaces.

Furthermore no damage to an aluminium or a stainless steel surface hasbeen observed when conducting the surface safety test method in hotconditions, i.e. bowling point temperature of the compositions tested,with compositions 4 and 5 for an exposure period time of 0.5 hours.

What is claimed is:
 1. An aqueous cleaning composition having a pH offrom about 0.1 to about 4.5, suitable for removing limescale deposits onhard surfaces, comprising from about 0.1% to about 30% of surfactant, ormixtures thereof; from about 0.5% to about 5% of sulfamic acid; and alevel of maleic acid to provide a weight ratio of maleic acid tosulfamic acid of from about 15:1 to about 3:51.
 2. A compositionaccording to claim 1 wherein the surfactant is selected from the groupconsisting of nonionic, anionic, cationic zwitterionic, amphotericsurfactants and mixtures thereof.
 3. A composition according to claim 1wherein the suffactant is a nonionic surfactant which is a condensationproduct of ethylene oxide with an alcohol, said alcohol having astraight alkyl chain comprising from about 6 to about 22 carbon atoms,said condensation product having a degree of ethoxylation of from about1 to about 15, or mixtures thereof.
 4. A composition according to claim1 wherein the surfactant is a cationic surfactant according to theformula R₁ R₂ R₃ R₄ N⁺ X⁻, wherein X is a counteranion, R₁ is a C₈ -C₂₀hydrocarbon chain and R₂, R₃ and R₄ are independently H or C₁ -C₄hydrocarbon chains.
 5. A composition according to claim 1 wherein thesurfactant is a mixture of a nonionic surfactant with a cationicsurfactant according to the formula R₁ R₂ R₃ R₄ N⁺ X⁻, wherein X is acounteranion, R₁ is a C₈ -C₂₀ hydrocarbon chain and R₂, R₃ and R₄ areindependently H or C₁ -C₄ hydrocarbon chains.
 6. A composition accordingto claim 1 wherein the pH is from about 0.1 to about
 3. 7. A compositionaccording to claim 1 wherein the pH is from about 0.5 to about
 2. 8. Acomposition according to claim 1 wherein the weight ratio of maleic acidto said acid is from about 8:1 to about 4:1.
 9. A process of treatinghard-surfaces, wherein a composition according to claim 1 is dispensedin its neat form from a container or used in a diluted form onto saidsurfaces, then left to act onto said surfaces and then removed byrinsing, thereby improving surface safety.
 10. A process according toclaim 9 wherein the hard-surfaces are metal surfaces.